Math: Fix [l/t]zcnt intrinsics on GCC

X64Context: Surround with architecture ifdef

src/xenia/base/x64_context.cc

@@ -10,10 +10,13 @@
 #include "xenia/base/x64_context.h"
 
 #include "xenia/base/assert.h"
+#include "xenia/base/platform.h"
 #include "xenia/base/string_util.h"
 
 namespace xe {
 
+#if XE_ARCH_AMD64
+
 // NOTE: this order matches 1:1 with the X64Register enum.
 static const char* kRegisterNames[] = {
     "rip",   "eflags", "rax",   "rcx",   "rdx",   "rbx",   "rsp",
@@ -59,4 +62,6 @@ void X64Context::SetValueFromString(X64Register reg, std::string value,
   assert_always(false);
 }
 
+#endif  // XE_ARCH_AMD64
+
 }  // namespace xe